Three basic manufacturing techniques are used in the manufacture of soft contact lenses. These are generally referred to as lathing (cutting both lens surfaces and edge on a pre-polymerised ‘button’), spin casting (using a single concave mould piece to form the front surface of the lens) and cast moulding (using a concave mould piece to form the front surface of the lens and a second convex mould pieces to form the back surface of the lens).
Lathing is a suitable process where there is a low batch size and a wide variety of lens powers and/or shapes required e.g. for toric lens manufacture. Spin casting is a suitable process for higher batch sizes, however, the ‘open’ surface not being in contact with a mould will be parabolic this being a ‘compromise’ profile to the generally spherical profile of the cornea. Cast moulding is a suitably process for very high volume manufacture and facilitates the precise profiling of both the front (by the concave mould surface) of the lens and back surface (by the convex mould surface) of the lens e.g. creating a bi-curve or even a tri-curve profile for optimum cornea fit.
There are also three basic contact lens packaging systems: a glass vial with bung and metal clip closure; a plastic ‘mould-cup’ with metallised foil seal; and an integrated plastic ‘mould-cup’ where one of the mould pieces used to form a cast lens is also used as the packing container generally sealed with metallised foil.
The health-care benefits of frequent lens replacement, e.g. monthly, bi-weekly or daily replacement, necessitate the lowest cost of lens manufacture and packaging. For example, the cost of vial packaging renders this system inappropriate on cost grounds for any frequent replacement modality of lens wear. Daily-disposable contact lenses are increasingly recognised as providing the healthiest modality of contact lens wear but the wearer's requirement of up to 730 contact lenses per year requires extremely low unit cost whilst ensuring high quality lens manufacture and high levels of on-eye comfort and visual acuity.
The optimum combination for meeting these stringent requirements would be the use of a cast-moulding process incorporating the use of one of the mould-pieces (either the concave or convex moulding) to form the lens-cup package.
In cast-moulding of soft contact lenses, which typically comprises curing a curable solution (of, typically, polymerisable monomers) in a lens-forming cavity formed between two mould halves, the common challenges include controlling the power of the contact lens being produced and edge formation. If the edge-formation is inconsistent, it may require cutting. If the edge formed is too discrete, it may cause discomfort for the user. A further problem in the cast-moulding of contact lenses is how to deal with monomer ‘shrinkage’, the inevitable reduction in volume in the curing stage. The monomer materials undergo volumetric shrinkage of at least 10% and typically between 10 and 20%. Failure to properly compensate for this shrinkage will result in unacceptably high wastage rates and/or poor quality products containing voids or bubbles.
There are several approaches that have been developed in the cast-moulding of contact lenses designed to enhance contact lens manufacture and in some cases to overcome one or more of the above problems.
GB-A-2006091 describes a method of manufacturing contact lenses by curing (or gelling) a contact lens-forming solution in a lens shaped space formed between a concave mould and a convex mould. This method is characterised by the mould cavity being an open mould and by over-filling the mould cavity with solution so as to form a reservoir of solution which can seep back into the mould cavity to allow for volumetric shrinkage during the curing (or gelling) process. A particular problem with this method, however, is that fluid within the channel (at the edges of the mould cavity), where fluid from the reservoir is intended to flow into the mould cavity during curing, tends to cure more rapidly (due to the confined volume) as compared with the main body of the lens cavity. A further problem is that post-curing, it is necessary to cut the cured ‘reservoir’ ring from the mould, or where it is selectively cured to cut or polish imperfectly formed edges. Rigid, inflexible materials, such as glass, are preferred for use as the mould halves.
WO-A-87/04390 describes a polyolefin mould for casting contact lenses. A mould cavity in which a monomer composition is placed for formation of a contact lens is formed between plastic male and female mould halves which cooperate by sliding fit and reach a final engagement position when a rigid (non-flexible) shoulder on one mould half (typically the female) engages with the other mould half to form a seal. The point of engagement of the rigid shoulder with the other mould half (preferably a rigid engagement) defines the radial diameter of the lens to be formed. At least one mould half, typically the male mould half, is formed with a diaphragm portion in which the material is sufficiently thin and flexible as to move toward the other mould half under the forces generated by the monomer shrinkage during curing. Such diaphragm behaviour of at least one mould half thereby compensates for the volumetric shrinkage during curing. The diaphragm behaviour is intended to avoid bubbles and voids in the resultant contact lenses. The rigid shoulder is preferably formed on the female mould half and preferably with a slight return, to ensure that the moulded lens remains in the female mould half when the mould halves are separated. WO-A-87/04390 further discloses that the female mould half retaining the cured lens may be used as a package for the hydrated lens by hydrating the lens and sealing a lid to the flange of the female mould half. A particular disadvantage of this system is that the diaphragm portion which flexes during curing is difficult to form in a manner which gives consistent curvature in the lenses (or to ensure that cavitation and bubbles in the lens are avoided). Further, in providing a return on a non-flexible shoulder, removal of the lens from the female mould half is difficult, even if hydrated in situ. The proposed mould arrangement is bulky (in order to ensure that a slide fit engagement can be achieved and to provide internal volume for later processing), which leads to significant material loss (in the male half that is disposed of) and resultant bulky packaging of contact lens.
U.S. Pat. No. 5,143,660 is an alternative arrangement for providing a mould half as a contact lens package and which arrangement also utilises diaphragm behaviour of the mould surfaces to compensate for monomer shrinkage during curing. In U.S. Pat. No. 5,143,660, the two mould halves cooperate to seal against a rigid shoulder by a sliding fit which seal defines a radial diameter of the lens. The curing stage is carried out at superatmospheric pressure to ensure that even deflection of the diaphragm surface (typically of each mould half) occurs in order to produce lenses of consistent and even curvature. The male mould half surface is formed with greater surface energy so as to allow the lens to remain on the male mould half after separation of the mould halves. The male mould half is provided with an annular wall (which is involved in the sliding fit) whereby a lid may be applied to the rim of the annular wall to form a package in which the lens is provided on the convex internal surface of the package, thereby presenting the lens in a manner that it can be removed by the user without touching the eye-contacting surface of the lens. Disadvantages of this system include the precision of manufacture necessary to ensure sealed sliding fit and diaphragm behaviour, the increased complexity of requiring superatmospheric pressure, the excess material required to provide the slide-fit engagement (and non-optical) portions of the mould half and the fact that the convention in contact lens use is for the user to be presented with a lens concave surface up.
EP-A-0383425 describes a contact lens mould arrangement having male and female mould halves in sliding fit cooperation in which the male mould half is provided with a shoulder that engages a cylindrical or frusto-conical portion of the female mould half adjacent the anterior lens surface-forming mould surface. During polymerisation, the male mould half may move toward the female mould half from its pre-engagement position defined by engagement of mating surfaces (flanges) formed on the distal edges of the cylindrical walls as a result of a hinging effect of the male mould half at the shoulder junction. Thereby the volumetric shrinkage may be compensated by the male mould half moving slightly toward the female mould half as a result of this lever effect.
Several documents describe another form of mould arrangement in which the curvature of the lens forming surfaces of the mould halves remains constant but the cavity volume (and radial diameter) changes during the curing by providing an annular flexible lip on the male or female mould half at the extremity of lens-forming cavity, which flexing of the lip allows the two mould halves to move closer together to compensate for monomer shrinkage. One example is GB-A-1575694 in which a piston-cylinder slide fit arrangement of rigid mould halves is provided to define a mould cavity, the edge region defined by the engagement of a flexible rim formed on the male or female mould half with the other mould half. During curing, the lip flexes (typically inward) allowing the mould halves to move closer to compensate for monomer shrinkage. WO-A-2004/076160 provides a similar slide-fit arrangement whereby a flexible annular lip is provided on a female mould half which engages with an annular abutment section of a male mould half thereby defining the edge of the contact lens. During curing, the lip may flex radially outwards along the abutment surface thereby increasing the radial dimension of the lens forming cavity whilst the two mould halves are drawn together. A particular disadvantage of the flexible lip arrangement is that the flexible lip can affect the seal resulting in reject lenses and the edge portions of resulting lenses tend to be inconsistent and in need cutting or polishing. Further, to ensure misalignment does not occur, the cylinder-piston arrangement of mould halves must be manufactured with a good deal of precision to ensure close and replicable fit.
The prior art thus suffers from disadvantages mentioned, which typically include one or more of inconsistency of lens edge-forming, reliability and consistency of lens curvature, complexity of manufacturing, requirements for precision in non-optical portions of mould-halves, degree of material waste and unsuitability for use of the lens-forming mould half in packaging.
The present inventors have found that a fundamentally new approach to cast moulding of contact lenses allows them to overcome many of the above problems, to utilise more efficient manufacturing processes and to effectively and efficiently utilise a mould half as a blister in contact lens packaging.